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Sectioned PDWA photo

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Scott_Hower

Scott_Hower

Luke Skywalker
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Can't remember who asked for this, but we had a discussion recently about the function of a PDWA. I promised to cut one open to show the guts. If you have ever wondered what is inside one of these evil things, well, now you know.

Yes, I know the price of a new one and before anyone gets excited, this one was scrap. One of the threaded brake line exits had a gouge in the bottom and refused to seal the flare. It gave me great satisfaction to cut it in half. Tool used was a Grizzly metal cutting bandsaw. Took all of 30 seconds to make it into a sandwich.

p2.jpg

p.jpg
 
<div class="ubbcode-block"><div class="ubbcode-header">Quote:]Yes, I know the price of a new one [/QUOTE]

One of the reasons I have a drawer full of used ones

......hmmmm, it looks a lot like a rear wheel adjuster!

One way to tell if its not working is to put the car in neutral & push it forward...if it won't move, its a good bet the thing is bad (that is , if your brakes are adjusted properly to include the hand brake & the clutch is allowing the trans/engine to get into neutral)
 
Cool....I banged one apart w/ a BFH once.....

But it wasn't as easy to visualize how it worked.
 
tony barnhill said:
One way to tell if its not working is to put the car in neutral & push it forward...if it won't move, its a good bet the thing is bad (that is , if your brakes are adjusted properly to include the hand brake & the clutch is allowing the trans/engine to get into neutral)
Click to expand...


I got well and truly fooled last year by BoneIdle's GT. Rear brakes seemed to be working poorly, SOME braking, no release... no fluid from the bleeders. "Ah-HA!" sez I, "Poxy fore-aft valve!"

Yanked it all apart, made a mess and found it to be FINE. What was "valving" was the rear flex-line. Quick R&R... I felt a bit foolish, BTW: THAT shoulda been the first place I looked. BTDT too many times before. /bcforum/images/%%GRAEMLIN_URL%%/blush.gif
 
Yep, Doc, whenever I think its the PDWA, I check everything else first...if all else is in working order, then I try the push test.
 
Excuse me gentlemen... I don't think the pressure differential warning actuator has any role in braking ability or regulating or compensating. This thing is just a switch. If there is a difference in pressure between the front circuit and the rear, the shuttle piston moves to the low pressure side, and the plastich switch probe is pushed up closing the contacts for the light to come on. That's it. I'm pretty certain that the shuttle piston can not block any of the ports. Sometimes they leak when the seals on the shuttle perish, then fluid comes out of the switch. It's meant to be dry in there. The switch doesn't seal. Yet folks replace the switch only, when there's a leak. When the system is opened, you should check that the shuttle is centered.

Cars that have compensators are different. The older versions (Fiat, Renault, etc.) worked by sensing a nose dive and would apply more rear brakes. New cars work with ABS and wheel speed sensors.

And Doc, a good test for blockage is toopen a bleed screw on the stuck brake. If fluid shoots out, it's likely the hose has collapsed.

Peter C.
 
<div class="ubbcode-block"><div class="ubbcode-header">Quote:]I don't think the pressure differential warning actuator has any role in braking ability or regulating or compensating.[/QUOTE]

I can guarantee you, from experience, that if that sucker isn't doing whatever its supposed to do, the car will not roll forward in neutral!

& I learned that the hard way.
 
tony barnhill said:
<div class="ubbcode-block"><div class="ubbcode-header">Quote:]I don't think the pressure differential warning actuator has any role in braking ability or regulating or compensating.
Click to expand...

I can guarantee you, from experience, that if that sucker isn't doing whatever its supposed to do, the car will not roll forward in neutral!

& I learned that the hard way. [/QUOTE]

So... what was the fix?
 
<div class="ubbcode-block"><div class="ubbcode-header">Quote:] Originally Posted By: tony barnhill
Quote:
I don't think the pressure differential warning actuator has any role in braking ability or regulating or compensating.


I can guarantee you, from experience, that if that sucker isn't doing whatever its supposed to do, the car will not roll forward in neutral!

& I learned that the hard way.


So... what was the fix? [/QUOTE]

Replaced the PDWA
 
<div class="ubbcode-block"><div class="ubbcode-header">Quote:]You didn't open the PWDA to see what happened?

You just replaced it?

..and opened the system .....
[/QUOTE]

Who cares what happened to it...nothing else I could do to the system would make the brakes work...to include flushing & adding new fluid, pads, shoes, rear wheel cylinders, hoses....& the car still wouldn't move - it was like the brakes were locked up.....& I couldn't see anything wrong with the brakes at the wheels!

....so, as a last resort, I took the PWDA off & when the car would - with it off & the system open - suddenly move, I put another PWDA on, refluided the system, bled it & the brakes worked....& still do!
 
At a point of catastrophic hydraulic failure the little shuttlecock is snapped to the failed side and mostly blocks the thru port on the valve body to limit fluid loss, but keeps the functioning side open by way of the cone shape. A PITA device, but more than just a lightswitch.
 
That's why I enjoy smashing the little bitz outta em. /bcforum/images/%%GRAEMLIN_URL%%/grin.gif /bcforum/images/%%GRAEMLIN_URL%%/hammer.gif
 
i could have fixed that port.......

i also note that my shuttle was different than that one, mine had a two grooves on either side, forming a rib in the center for the switch to sit on when centered. i can imagine that a failed one could possible hold pressure to one set of brakes, keeping them from releasing.

mark
 
PeterC said:
Excuse me gentlemen... I don't think the pressure differential warning actuator has any role in braking ability or regulating or compensating. This thing is just a switch. If there is a difference in pressure between the front circuit and the rear, the shuttle piston moves to the low pressure side, and the plastich switch probe is pushed up closing the contacts for the light to come on. That's it.

I'm pretty certain that the shuttle piston can not block any of the ports. Sometimes they leak when the seals on the shuttle perish, then fluid comes out of the switch. It's meant to be dry in there. The switch doesn't seal. Yet folks replace the switch only, when there's a leak. When the system is opened, you should check that the shuttle is centered.
Click to expand...

Ah jeez, now I remember why I did this.

BY DESIGN, the tandem master cylinder splits the brake circuit and the PDWA keeps it that way and senses when it is not.

The F/R ports are already split (and kept split) by the plunger inside the PDWA. If they are not, the hydraulic pressure inside would just equalize and the plunger wouldn't move. Think about it. It HAS TO split the circuit or it wouldnt work in the first place. The pressure differential due to the SEALING of the front from rear circuits is what causes the pluger to move and actuate the switch in the first place. It isnt like the plunger is suspended in fluid and just swooshes around to the low pressure side.

No one is stating the ports "get blocked". They are already "blocked" (isolated) by virtue of the seals on the shuttle. Besides, the device does not need to "block any ports" as the low pressure side is useless in the event of a leak except for total fluid loss, which the device serves to prevent. Port blockage is irrelevant to a system that won't hold pressure. Circuit seperation is all that matters.

I will argue this all day long, as that's the whole point of the device in the first place. All it does is keep the front circuit seperate from the back and indicate if there is a differential. It is a switch and a fluid seperator, but not a proportioning valve.
 
tdskip said:
Hey Scott - just a heads up that I'll be sending all the kids sciences project out to you for cross-sectioning...
Click to expand...

Bring it on, Tom "... My old man's got an awesome set of tools." /bcforum/images/%%GRAEMLIN_URL%%/devilgrin.gif
 
<div class="ubbcode-block"><div class="ubbcode-header">Quote:]It is a switch and a fluid seperator, but not a proportioning valve. [/QUOTE]

Scott, think most of us agree with you about its purpose...what happens when it goes bad appears to be the point of discussion.
 
Tony-

I suspect what happens when it goes bad: high pressure brake fluid transfers from the "bad side" to the "good" (gets pumped through the seals on MC pressure) and one of the wonky seals keeps it that way.

Stated another way... The MC has enough pressure to push the fluid past one (or both) bad seals, and one of the two seals has enough grip to hold it there while the other does not. This locks one circuit but not the other. A dead giveaway is fluid up through the switch threads which renders the switch useless (it wont light).
 
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